Sintered body of silicon nitride-based ceramics

ABSTRACT

There are disclosed a sintered body of ceramics, comprising 0.1 to 10% by weight of yttrium oxide; 0.1 to 10% by weight of aluminum oxide; 0.1 to 10% by weight of aluminum nitride; 0.1 to 5% by weight of at least one silicide selected from the group consisting of magnesium silicide, calcium silicide, titanium silicide, vanadium silicide, chromium silicide, manganese silicide, zirconium silicide, niobium silicide, molybdenum silicide, tantalum silicide and tungsten silicide; and the balance being silicon nitride, and a process for producing a sintered body of ceramics, which comprises molding a powder mixture of the same composition, and sintering the resultant molded compact in a non-oxidative atmosphere. 
     According to the present invention, it is possible to manufacture sintered body having high density and excellent impact resistance.

This invention relates to a sintered body of ceramics composed primarily of silicon nitride and preparation thereof. More particularly, it pertains to a sintered body of ceramics which is high in density, excellent in mechanical strength and impact resistance and also low in the degree of lowering of mechanical strength even under an oxidizing atmosphere for prolonged time at a high temperature, and to a process for producing the same.

BACKGROUND OF THE INVENTION

A sintered body of ceramics, which is excellent in thermal properties and has high density, is now attracting attention as a frontier material among various structural materials in widely varying fields of industries. As a typical example, there is a sintered body of silicon nitride.

In the art of production of silicon nitride sintered bodies, there have generally been adopted the reaction sintering method, the hot press method and the ordinary sintering method.

Among them, the reaction sintering method is a method in which metallic silicon powders are molded previously into a desired shape, which is in turn heated gradually in an atmosphere of nitrogen or ammonia gas thereby to be converted to a nitride simultaneously with sintering (see, for example, Proc. of DARPA/NAVSEA, Ceramic Gas Turbine Demonstration Engine Program Review, ed. by W. Fairbanks and R. W. Rice, MCIC Report, March 1978 [MCIC-78-36]).

The hot press method comprises adding a sintering aid (e.g. Y₂ O₃, MgO, Al₂ O₃) to the powders of silicon nitride (Si₃ N₄), and sintering the resultant mixture in a certain mold (e.g. a mold of graphite) at a high temperature of 1700° to 1800° C. by application of a pressure of 500 kg/cm² (see, for example, Norton Catalog). According to this method, there can be obtained a sintered product having a high density with greater mechanical strength and also having excellent impact resistance as well as excellent thermal properties with a small degree of lowering of mechanical strength under an oxidizing atmosphere at higher temperatures. However, while this method involves the drawback of difficult fabrication of a complicated and large size sintered body on one hand, it is also disadvantageously inferior in capability of bulk production.

On the other hand, the ordinary sintering method comprises previously moding Si₂ N₄ powders and a sintering aid together with a binder such as paraffin, and then sintering by heating the resultant compact as such without hot press under a non-oxidative atmosphere (see, for example, GTE Sylvania Catalog or Carborundum Co. Technical Sheet). According to this method, however, it is difficult to obtain a sintered body having a high density with excellent mechanical strength and impact resistance.

SUMMARY AND OBJECTS OF THE INVENTION

In view of the state of the art as outlined above, the present inventors have made various investigations on the above ordinary sintering method and consequently proposed improved ordinary method comparable with the hot press method, which is capable of producing a high density sintered body excellent in mechanical strength and impact resistance (Japanese Provisional Patent Publications Nos. 113674/1980 and 116670/1980).

However, the silicon nitride sintered bodies obtained according to these method proved to be still not satisfactory in resistance to lowering of mechanical strength under a high temperature oxidizing atmosphere.

The present inventors have made further extensive studies on the above point and at last accomplished the present invention.

The object of the present invention is to provide a sintered body of ceramics having a high density and excellent impact resistance, which is also small in lowering of mechanical strength under an oxidative atmosphere for prolonged time at a high temperature, and above all a sintered body of ceramics composed primarily of silicon nitride and a process for producing the same.

That is, the sintered body of ceramics according to the present invention comprises 0.1 to 10% by weight of yttrium oxide (Y₂ O₃); 0.1 to 10% by weight of aluminum oxide (Al₂ O₃); 0.1 to 10% by weight of aluminum nitride (AlN); 0.1 to 5% by weight of at least one silicide selected from the group consisting of magnesium silicide (Mg₂ Si), calcium silicide (CaSi₂), titanium silicide (TiSi₂), vanadium silicide (VSi₂), chromium silicide (CrSi₂), manganese silicide (MnSi), zirconium silicide (ZrSi₂), niobium silicide (NbSi₂), molybdenum silicide (MoSi₂), tantalum silicide (TaSi₂) and tungsten silicide (WSi₂); and the balance being silicon nitride (Si₃ N₄).

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The sintered body of ceramics according to the present invention is constituted primarily of Si₃ N₄, which may preferably contained in an amount of 70% by weight or more, more preferably 80 to 90% by weight. There may be employed either α-phase type or β-phase type Si₂ N₄, or mixture thereof, but preferably α-phase type.

Both Y₂ O₃ and Al₂ O₃ function as sintering promoters. Both of these components are contained in amounts of 0.1 to 10% by weight. Preferably, both Y₂ O₃ and Al₂ O₃ may be contained in amounts of 0.5 to 8% by weight, respectively. These components, at the levels exceeding 10% by weight, will undesirably lower the mechanical strength and impact resistance of the resultant sintered body. It is generally preferred that the sum of the contents of the both components should be within the range from 3 to 15% by weight, more preferably 4 to 10% by weight.

AlN has the function to suppress evaporation of the primary component Si₃ N₄ during sintering procedure and it also contributes to promotion of sintering as a whole by formation of a liquid phase useful for sintering through the reaction with other components. The content of AlN may be 0.1 to 10% by weight, preferably 0.5 to 8% by weight. At a level in excess of 10% by weight, the resultant sintered body will be lowered in mechanical strength and impact resistance.

The silicide such as Mg₂ Si, CaSi₂, TiSi₂, VSi₂, CrSi₂, MnSi, ZrSi₂, NbSi₂, MoSi₂, TaSi₂ and WSi₂ all function not only to aid the sintering promoters of Y₂ O₃ and Al₂ O₃ but also to protect resultant sintered body against lowering in mechanical strength, since these silicides will form protective coated films with great oxidation resistance on the surface of the sintered body under an oxidative atmosphere at a high temperature. In particular, MoSi₂, CrSi₂, ZrSi₂ and TiSi₂ will contribute very much to that effect. These silicides may be contained in an amount of 0.1 to 5% by weight, preferably 0.25 to 3% by weight. But, when the content of these silicides exceed 5% by weight, the sintered body may contrariwise be lowered in mechanical strength and impact resistance to a disadvantage.

The process for producing the sintered body of ceramics according to present invention comprises molding a powder mixture comprising 0.1 to 10% by weight of yttrium oxide (Y₂ O₃) powders; 0.1 to 10% by weight of aluminum oxide (Al₂ O₃) powders; 0.1 to 10% by weight of aluminum nitride (AlN) powders; 0.1 to 5% by weight of at least one powdery silicide selected from the group consisting of magnesium silicide (Mg₂ Si), calcium silicide (CaSi₂), titanium silicide (TiSi₂), vanadium silicide (VSi₂), chromium silicide (CrSi₂), manganese silicide (MnSi), zirconium silicide (ZrSi₂), niobium silicide (NbSi₂), molybdenum silicide (MoSi₂), tantalum silicide (TaSi₂) and tangusten silicide (WSi₂); and the balance of silicon nitride (Si₃ N₄) powders, and sintering the resultant compact in a non-oxidative atmosphere.

In the process according to the present invention, these components may be mixed by means of a pulverizing mixer, using a solvent such as n-butyl alcohol.

The powder mixture thus prepared is admixed with a binder such as paraffin, etc. and molded into a desired shape by application of a suitable pressure.

The resultant compact is then sintered by heating at 1500° to 1900° C., preferably at 1600° to 1800° C., in a non-oxidative atmosphere to be converted to a sintered body. As a non-oxidative atmosphere, there may be mentioned a non-oxidative atmosphere of nitrogen, argon, etc. No oxidative atmosphere can be used, since Si₃ N₄ will be oxidized to SiO₂ in such an atmosphere. The sintering may also be effected under a hot press condition by application of a pressure of 50 to 500 kg/cm², or under the pressurized condition in a non-oxidative gas atmosphere. Alternatively, the characteristics of the sintered product will not be changed if sintering may be effected by the ordinary sintering method, followed by sintering under pressurized atmosphere or by HIP method (Hot Isostatic Press method) (see, for example, H. Larker et al., SAE Pape 770335, Detroit, February 1977).

The present invention is described in further detail by referring to the following Examples and Reference examples.

EXAMPLES 1-21

There were prepared 21 kinds of powdery mixtures as Examples according to the present invention by formulating the respective components at predetermined proportions (% by weight) as indicated in Table 1 and then, after addition of n-butyl alcohol to each formulation, blending the mixtures in a rubber-lined ball mill for 24 hours, respectively. The powders of Si₃ N₄ comprises powders with an average particle diameter of 1.2μ containing 85% of α-phase type Si₃ N₄. The powders of Y₂ O₃, Al₂ O₃, AlN and various silicide are average diameters of 1.0μ, 0.5μ, 1.5μ and 1.0μ, respectively.

After 7% by weight of paraffin was further added to each of the resultant powdery mixtures, each mixture was molded under a molding pressure of 700 kg/cm² into a plate of 60 mm in length, 40 mm in width and 10 mm in thickness. Each compact obtained was subjected to heat treatment to remove the paraffin by pyrolysis and then sintered at 1750° C. under a nitrogen gas stream at 3 liter/min.

Each sintered body was tested for relative density, flexural strength under room temperature, flexural strength under room temperature after treatment in air at 1000° C. for 1000 hours and in air at 1200° C. for 1000 hours, respectively, and impact resistance.

The results obtained are shown as Examples 1-21 in Table 1. The respective measurement items were conducted according to the following methods.

Relative density (%):

indicated in terms of the relative percent of found density relative to the theoretical density calculated by the composition ratio.

Flexural strength (kg/mm²):

measured according to the three point flexural strength test, with a strip size of 3×3×30 mm, a cross-head speed at 0.5 mm/min., a span of 20 mm, at room temperature; four test strips were measured for each sample.

Impact resistance (ΔT; °C.):

a strip with the same shape as that for measurement of flexural strength was heated to a certain temperature, followed by quenching by throwing into water, and the presence of cracks generated on the test strip was observed; impact resistance is indicated in terms of ΔT the difference between the heating temperature when crack was generated and the temperature of water.

REFERENCE EXAMPLES 1-11

There was obtained 11 kinds of sintered bodies according to the same procedure as in Examples except for using the starting materials having the compositions as indicated in Table 1.

Each sintered body obtained was subjected to measurement of relative density, flexural strength and impact resistance according to the same methods as described in the above Examples.

The results are also shown as Reference examples 1-11 in Table 1.

                                      TABLE 1                                      __________________________________________________________________________     Starting material composition (wt. %)                                                                          Flexural strength (kg/mm.sup.2)                                                    1000° C.                                                                       1200° C.                                                Relative                                                                            Before                                                                             Oxidation                                                                             Oxidation                                               Other  density                                                                             oxi-                                                                               treatment                                                                             treatment                                                                             Δ T                    Sample                                                                               Si.sub.3 N.sub.4                                                                   Y.sub.2 O.sub.3                                                                   Al.sub.2 O.sub.3                                                                   AlN                                                                               component                                                                             (%)  dation                                                                             after 1000 hr.                                                                        after 1000 hr.                                                                        (°C.)                 __________________________________________________________________________     Example 1                                                                            78  10 2   5  MoSi.sub.2                                                                          5 99.6 85  80     74     700                          Example 2                                                                            86  5  3.5 3.5                                                                               MoSi.sub.2                                                                          2 99.0 100 90     82     750                          Example 3                                                                            87.5                                                                               5  3.5 3.5                                                                               MoSi.sub.2                                                                          0.5                                                                              98.7 90  86     75     825                          Example 4                                                                            86  5  3.5 3.5                                                                               CrSi.sub.2                                                                          2 98.8 93  88     80     750                          Example 5                                                                            86  5  3.5 3.5                                                                               ZrSi.sub.2                                                                          2 98.9 99  92     80     800                          Example 6                                                                            86  5  3.5 3.5                                                                               TiSi.sub.2                                                                          2 99.2 95  86     81     800                          Example 7                                                                            86  5  3.5 3.5                                                                               Mg.sub.2 Si                                                                         2 98.5 89  80     71     750                          Example 8                                                                            86  5  3.5 3.5                                                                               CaSi.sub.2                                                                          2 99.5 87  76     70     700                          Example 9                                                                            86  5  3.5 3.5                                                                               VSi.sub.2                                                                           2 98.8 92  80     72     775                          Example 10                                                                           86  5  3.5 3.5                                                                               MnSi 2 98.0 90  81     70     700                          Example 11                                                                           86  5  3.5 3.5                                                                               NbSi.sub.2                                                                          2 99.7 90  85     72     775                          Example 12                                                                           86  5  3.5 3.5                                                                               TaSi.sub.2                                                                          2 99.4 95  90     75     750                          Example 13                                                                           86  5  3.5 3.5                                                                               WSi.sub.2                                                                           2 98.8 91  81     73     775                          Example 14                                                                           86  5  3.5 3.5                                                                               MoSi.sub.2                                                                          1 99.2 100 95     86      800                                             CrSi.sub.2                                                                          1                                                     Example 15                                                                           86  5  3.5 3.5                                                                               ZrSi.sub.2                                                                          0.5                                                                              99.8 102 98     85     800                                              TiSi.sub.2                                                                          0.5                                                                       VSi.sub.2                                                                           0.5                                                                       NbSi.sub.2                                                                          0.5                                                   Example 16                                                                           84  2  5   7  MoSi.sub.2                                                                          2 99.2 88  80     71     725                          Example 17                                                                           87  10 2   0.5                                                                               ZrSi.sub.2                                                                          0.5                                                                              99.3 98  90     75     700                          Example 18                                                                           88.5                                                                               5  0.5 5  MoSi.sub.2                                                                          1 98.0 90  88     82     775                          Example 19                                                                           82.5                                                                               0.5                                                                               10  2  TiSi.sub.2                                                                          5 96.7 85  84     80     700                          Example 20                                                                           82  5  1   10 CrSi.sub.2                                                                          2 97.5 86  80     72     700                          Example 21                                                                           86.25                                                                              5  5   3.5                                                                               CrSi.sub.2                                                                          0.25                                                                             98.9 98  90     81     750                          Reference                                                                            87.5                                                                               5  3.5 3.5                                                                               TiO.sub.2                                                                           0.5                                                                              98.7 100 80     58     800                          example 1                                                                      Reference                                                                            87.5                                                                               5  3.5 3.5                                                                               MgO  0.5                                                                              98.9 95  60     45     700                          example 2                                                                      Reference                                                                            87.5                                                                               5  3.5 3.5                                                                               ZrO.sub.2                                                                           0.5                                                                              98.0 90  64     47     725                          example 3                                                                      Reference                                                                            86  5  3.5 3.5                                                                               TiO.sub.2                                                                           2 99.6 99  75     55     700                          example 4                                                                      Reference                                                                            88  5  5   -- MoSi.sub.2                                                                          2 94.8 70  65     61     650                          example 5                                                                      Reference                                                                            85  5  --  5  MoSi.sub.2                                                                          5 82.8 49  45     42     675                          example 6                                                                      Reference                                                                            80  -- 10  5  CrSi.sub.2                                                                          5 75.3 45  43     40     675                          example 7                                                                      Reference                                                                            80  5  3.5 3.5                                                                               MoSi.sub.2                                                                          8 99.8 72  70     68     600                          example 8                                                                      Reference                                                                            79  15 2   2  ZrSi.sub.2                                                                          2 99.7 70  65     58     600                          example 9                                                                      Reference                                                                            76  10 10  2  TiSi.sub.2                                                                          2 99.0 80  72     63     500                          example 10                                                                     Reference                                                                            76  2  5   15 MoSi.sub. 2                                                                         2 97.5 76  70     60     525                          example 11                                                                     __________________________________________________________________________

As seen from Table 1, the sintered bodies obtained according to the process of the present invention (Examples 1-21) all have high densities with relative densities being 95% or higher relative to the theoretical density, and also have great flexural strengths of 85 kg/cm² or more and high impact resistance in terms of ΔT of 700° C. or more. Above all, it has been found that the lowering of flexural strength is very small even after the oxidation treatment at 1000° C. and 1200° C. for 1000 hours, respectively.

EXAMPLES 22-26

Of the sintered bodies obtained in the foregoing examples, 5 kinds of sintered bodies of Examples 3, 4, 6, 14 and 15 were further subjected to sintering treatment according to the HIP method in an atmosphere of nitrogen at 1750° C. under a pressure of 1000 to 3000 atm.

Each sintered body obtained was tested for relative density, flexural strength and impact resistance as the same as in Examples 1-21 to obtain the results as shown in Table 2.

REFERENCE EXAMPLES 12-15

Of the sintered bodies obtained in the foregoing Reference examples, 4 kinds of sintered bodies of Reference examples 1, 2, 8 and 11 were further subjected to sintering treatment according to the HIP method similarly as in Examples 22-26, and each sintered body obtained was tested for relative density, flexural strength and impact resistance to obtain the results which are also shown in Table 2.

                                      TABLE 2                                      __________________________________________________________________________                                  Flexural strength (kg/mm.sup.2)                                                    1000° C.                                                                       1200° C.                                                Relative                                                                            Before                                                                             oxidation                                                                             oxidation                                    Starting material composition (wt. %)                                                            density                                                                             oxi-                                                                               treatment                                                                             treatment                                                                             Δ T                       Sample                                                                               Example or Reference example No.                                                                 (%)  dation                                                                             after 1000 hr.                                                                        after 1000 hr.                                                                        (°C.)                    __________________________________________________________________________     Example 22                                                                           Example 3         99.9<                                                                               110 105    85     875                             Example 23                                                                           Example 4         99.9<                                                                               115 109    90     800                             Example 24                                                                           Example 6         99.9<                                                                               115 102    88     850                             Example 25                                                                            Example 14       99.9<                                                                               113 108    92     850                             Example 26                                                                            Example 15       99.9<                                                                               118 110    90     850                             Reference                                                                            Reference example 1                                                                              99.9<                                                                               115  93    65     850                             example 12                                                                     Reference                                                                            Reference example 2                                                                              99.9<                                                                               110  71    53     700                             example 13                                                                     Reference                                                                            Reference example 8                                                                              99.9<                                                                                75  72    69     600                             example 14                                                                     Reference                                                                             Reference example 11                                                                            99.9<                                                                                84  80    72     550                             example 15                                                                     __________________________________________________________________________

As described in detail above, the process of the present invention requires no hot press and therefore is very suitable for bulk production. Moreover, according to the process of the present invention, it is possible to manufacture sintered bodies having high density and excellent impact resistance, with small degree of lowering of mechanical strength under an oxidative atmosphere at a high temperature, to a great advantage in industrial application. 

We claim:
 1. A sintered body of ceramics having high density, high flexural strength, and high impact resistance, comprising 0.1 to 10% by weight of yttrium oxide (Y₂ O₃); 0.1 to 10% by weight of aluminum oxide (Al₂ O₃); 0.1 to 10% by weight of aluminum nitride (AlN); 0.1 to 5% by weight of at least one silicide selected from the group consisting of magnesium silicide (Mg₂ Si), calcium silicide (CaSi₂), titanium silicide (TiSi₂), vanadium silicide (VSi₂), chromium silicide (CrSi₂), manganese silicide (MnSi), zirconium silicide (ZrSi₂), niobium silicide (NbSi₂), molybdenum silicide (MoSi₂), tantalum silicide (TaSi₂), and tungsten silicide (WSi₂); and the balance being silicon nitride (Si₃ N₄).
 2. A sintered body of ceramics according to claim 1, wherein said sintered body comprises 0.5 to 8% by weight of yttrium oxide; 0.5 to 8% by weight of aluminum oxide; 0.5 to 8% by weight of aluminum nitride; 0.25 to 3% by weight of at least one silicide selected from the group consisting of magnesium silicide, calcium silicide, titanium silicide, vanadium silicide, chromium silicide, manganese silicide, zirconium silicide, niobium silicide, molybdenum silicide, tantalum silicide and tungsten silicide; and the balance being silicon nitride.
 3. A sintered body of ceramics according to claim 1, produced by the process comprising the steps of molding a powder mixture comprising 0.1 to 10% by weight of yttrium oxide (Y₂ O₃) powders; 0.1 to 10% by weight of aluminum oxide (Al₂ O₃) powders; 0.1 to 10% by weight of aluminum nitride (AlN) powders; 0.1 to 5% by weight of at least one powdery silicide selected from the group consisting of magnesium silicide (Mg₂ Si), calcium silicide (CaSi₂), titanium silicide (TiSi₂), vanadium silicide (VSi₂), chromium silicide (CrSi₂), manganese silicide (MnSi), zirconium silicide (ZrSi₂), niobium silicide (NbSi₂), molybdenum silicide (MoSi₂), tantalum silicide (TaSi₂), and tungsten silicide (WSi₂); and the balance being silicon nitride (Si₃ N₄) powders; and sintering the resultant compact in a non-oxidative atmosphere at a temperature between about 1500° C. and 1900° C.
 4. A sintered body of ceramics according to claim 2, produced by the process comprising the steps of molding a powder mixture comprising 0.5 to 8% by weight of yttrium oxide powders; 0.5 to 8% by weight of aluminum oxide powders; 0.5 to 8% by weight of aluminum nitride powders; 0.25 to 3% by weight of at least one powdery silicide selected from the group consisting of magnesium silicide, calcium silicide, titanium silicide, vanadium silicide, chromium silicide, manganese silicide, zirconium silicide, niobium silicide, molybdenum silicide, tantalum silicide, and tungsten silicide; and the balance being silicon nitride powders; and sintering the resultant compact in a non-oxidative atmosphere at a temperature between about 1500° C. and 1900° C.
 5. A sintered body of ceramics according to claim 1, wherein the silicon nitride comprises at least about 70% by weight of the ceramic.
 6. A sintered body of ceramics according to claim 1, wherein the silicon nitride comprises between about 80 and 90% by weight of the ceramic.
 7. A sintered body of ceramics according to claim 1, wherein the silicon nitride comprises a major proportion of alpha-phase silicon nitride.
 8. A sintered body of ceramics according to claim 1, wherein the combined content of the yttrium oxide and aluminum oxide is between about 3 and 15% by weight.
 9. A sintered body of ceramics according to claim 1, wherein the combined content of the yttrium oxide and aluminum oxide is between about 4 and 10% by weight.
 10. A sintered body of ceramics according to claim 1, having a relative density of at least about 95% relative to the theoretical density, a flexural strength of at least about 85 kg/cm² and an impact resistance in terms of ΔT of at least 700° C. 